The city’s Education Department announced the new effort in a letter sent home to the parents of schoolchildren.

“Out of an abundance of caution and because ensuring the safety of our students and staff is our top priority, the D.O.E. will retest all school buildings based on the new protocol this winter,” the letter said. It also assured parents that “our drinking water is of the highest quality in the world.”

The new tests will be performed in accordance with state regulations issued this fall that mandate regular testing of water in schools and that discourage the city’s earlier testing protocol, in which the water was run for two hours the night before sampling.

The practice, known as prestagnation flushing, can clean the pipes of soluble lead and lead particles, possibly distorting the results.

The tests in the spring found that less than 1 percent of the samples had lead concentrations that exceeded the Environmental Protection Agency’s “action level” of 15 parts per billion.

However, since the pipes had been flushed, many parents and environmental advocates expressed concern that the results could not be relied upon.

The issue of lead in drinking water has long been a concern, especially in older buildings.

However, the number of children with lead poisoninghas declined in New York City since 2005, according to a 2015 report by the city’s Department of Health and Mental Hygiene. In 2014, 840 children younger than 6 were newly identified with blood lead levels of at least 10 micrograms per deciliter — a threshold the Centers for Disease Control and Prevention used to use as their “level of concern” — down from 2,705 children in 2005.

A version of this article appears in print on , on Page A25 of the New York edition with the headline: Water at Schools to Be Retested for Lead. Order Reprints | Today’s Paper | Subscribe